Filters for canceling multiple noise sources in borehole electromagnetic telemetry system

ABSTRACT

An electromagnetic borehole telemetry system providing improved signal to noise ratio. Adaptive filters use noise channels as references to remove noise from the signal channel. Multiple noise channels are coupled to series connected adaptive filters for removing each noise source from the signal channel. The order of noise removal is selected to remove the most significant first.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/923,184, entitled “Motion Sensor for Noise Cancellation in BoreholeElectromagnetic Telemetry System”, filed on the same date as thisapplication by the present inventors and assigned to the same assignee,which is hereby incorporated by reference for all purposes.

This application is related to U.S. Pat. No. 6,657,597, issued on Dec.2, 2003, entitled “Directional Signal and Noise Sensors for BoreholeElectromagnetic Telemetry System”, by the present inventors and assignedto the same assignee, which is hereby incorporated by reference for allpurposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

This invention relates to a borehole electromagnetic telemetry system,and in particular to filters which use multiple noise references tocancel noise in the signal channel.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is describedin connection with transmitting downhole data to the surface duringmeasurements while drilling (MWD), as an example. It should be notedthat the principles of the present invention are applicable not onlyduring drilling, but throughout the life of a wellbore including, butnot limited to, during logging, testing, completing and production. Theprinciples are also applicable to transmission of signals from thesurface to downhole equipment.

Heretofore, in this field, a variety of communication and transmissiontechniques have been attempted to provide real time data from thevicinity of the bit to the surface during drilling. The utilization ofMWD with real time data transmission provides substantial benefitsduring a drilling operation. For example, continuous monitoring ofdownhole conditions allows for an immediate response to potential wellcontrol problems and improves mud programs.

Measurement of parameters such as bit weight, torque, wear and bearingcondition in real time provides for more efficient drilling operations.In fact, faster penetration rates, better trip planning, reducedequipment failures, fewer delays for directional surveys, and theelimination of a need to interrupt drilling for abnormal pressuredetection is achievable using MWD techniques.

At present, there are four major categories of telemetry systems thathave been used in an attempt to provide real time data from the vicinityof the drill bit to the surface; namely, mud pressure pulses, insulatedconductors, acoustics and electromagnetic waves.

In a mud pressure pulse system, the resistance of mud flow through adrill string is modulated by means of a valve and control mechanismmounted in a special drill collar near the bit. This type of systemtypically transmits at 1 bit per second as the pressure pulse travels upthe mud column at or near the velocity of sound in the mud. It is wellknown that mud pulse systems are intrinsically limited to a few bits persecond due to attenuation and spreading of pulses.

Insulated conductors, or hard wire connection from the bit to thesurface, is an alternative method for establishing downholecommunications. This type of system is capable of a high data rate andtwo way communication is possible. It has been found, however, that thistype of system requires a special drill pipe and special tool jointconnectors which substantially increase the cost of a drillingoperation. Also, these systems are prone to failure as a result of theabrasive conditions of the mud system and the wear caused by therotation of the drill string.

Acoustic systems have provided a third alternative. Typically, anacoustic signal is generated near the bit and is transmitted through thedrill pipe, mud column or the earth. It has been found, how ever, thatthe very low intensity of the signal which can be generated downhole,along with the acoustic noise generated by the drilling system, makessignal detection difficult. Reflective and refractive interferenceresulting from changing diameters and thread makeup at the tool jointscompounds the signal attenuation problem for drill pipe transmission.

The fourth technique used to telemeter downhole data to the surface usesthe transmission of electromagnetic waves through the earth. A currentcarrying downhole data signal is input to a toroid or collar positionedadjacent to the drill bit or input directly to the drill string. When atoroid is utilized, a primary winding, carrying the data fortransmission, is wrapped around the toroid and a secondary is formed bythe drill pipe. A receiver is connected to the ground at the surfacewhere the electromagnetic data is picked up and recorded. It has beenfound, however, that in deep or noisy well applications, conventionalselectromagnetic systems are unable to generate a signal with sufficientintensity to be recovered at the surface.

In general, the quality of an electromagnetic signal reaching thesurface is measured in terms of signal to noise ratio. As the ratiodrops, it becomes more difficult to recover or reconstruct the signal.While increasing the power of the transmitted signal is an obvious wayof increasing the signal to noise ratio, this approach is limited bybatteries suitable for the purpose and the desire to extend the timebetween battery replacements. It is also known to pass band filterreceived signals to remove noise out of the frequency band of the signaltransmitter. These approaches have allowed development of commercialborehole electromagnetic telemetry systems which work at data rates ofup to four bits per second and at depths of up to 4000 feet withoutrepeaters in MWD applications. It would be desirable to transmit signalsfrom deeper wells and with much higher data rates which will be requiredfor logging while drilling, LWD, systems.

SUMMARY OF THE INVENTION

The present invention provides apparatus which improves the signal tonoise ratio in an electromagnetic telemetry system which telemeters databetween a borehole and the surface of the earth. A receiver includes anoise canceller which uses a reference noise channel to remove noisefrom a received signal. In a system with multiple noise sensors andnoise channels, a plurality of noise cancellers are used, with eachcanceller using one of the noise channels to remove noise from thesignal channel. In a preferred form, the cancellers include adaptivefilters and the filters are connected in series.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an oil well drilling rig and a wellboreelectromagnetic telemetry system in use while a well is being drilled.

FIG. 2 is a block diagram of an adaptive filter used to remove noisefrom a received electromagnetic signal.

FIG. 3 is a more detailed block diagram of the filter of FIG. 2 and amodel of signal and noise transmission paths.

FIG. 4 is a block diagram illustrating the structure of an adaptivetransversal filter.

FIG. 5 is a block diagram illustrating a filter tap coefficientalgorithm for the filter of FIG. 4.

FIG. 6 is a block diagram of a three-axis magnetometer and apparatus forbeam steering the magnetometer to alignment for optimal reception ofelectromagnetic radiation from a noise source.

FIG. 7 is a block diagram of a three-axis magnetometer and apparatus forbeam steering the magnetometer to alignment for optimal reception of atelemetry signal generated downhole.

FIG. 8 is a block diagram of a system for combining multiple noisechannels and removing the combined noise from a signal channel with anadaptive filter.

FIG. 9 is a block diagram of a system for removing multiple noisesources from a signal channel by use of multiple adaptive filters inseries.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a wellbore electromagnetic, EM, telemetrysystem will be described. A drill rig 10 is shown driving a drill pipe12 in a wellbore 14. The drill pipe 12 has a drill bit 16 on its lowerend. A motor 18 on the rig 10 represents an electric motor which mayrotate the drill pipe 12 and also represents other motors which would beused, e.g. to pump mud through the drill pipe 12. The drilling mud maybe used to drive a mud motor located just above drill bit 16.

An electronics package 20 is positioned within the drill pipe 12 neardrill bit 16. The electronics package includes sensors for measuringparameters, such as pressure and temperature, and a transmitter fortelemetering the information to the surface location of the well. Anymeans of transmitting an electromagnetic signal may be used. In thisfigure, the package 20 is shown driving an electric current into twosections of drill pipe 12 separated by an insulating section 22.Alternatively a toroidal core may be positioned around drill pipe 12 andthe package 20 may drive a winding on the core to generate an equivalenttransmitted electromagnetic signal. The toroidal core is usually anintegral part of a section of drill pipe or a drill collar to protectthe core from damage.

A transmitted electromagnetic signal is represented by lines of current24 and equipotential lines 26. This signal is detected at the surfacelocation of the well and coupled to a signal processor 28. The signalmay be detected in several ways. Electrical connections 30 may be madebetween a surface casing 32 and an electrode 34 implanted into thesurface of the earth some distance from the casing 32. The electricfield (E field) of the transmitted signal produces a voltage betweencasing 32 and electrode 34. This E field detector may be considered adirectional antenna which detects a horizontal component of a potentialdifference arising from the electric field of an EM signal. Thispotential difference may be amplified by an amplifier 36 and thencoupled to signal processor 28.

The magnetic field component of the transmitted EM signal 24, 26 mayalso be detected. A magnetometer 38 may be positioned in a locationselected to receive the transmitted EM signal. The detected magneticfield may be coupled to amplifier 36 and used as the signal channel, ormay be combined with the electrical signal from lines 30.

In the preferred embodiments, a plurality of sensors are used to detectvarious noise sources which generate EM noise. There are a number ofsources of EM noise which is also detected by the signal sensors, e.g.sensor 38, and which therefore reduces the signal to noise ratio of thesignal channel coupled to signal processor 28. The outputs from thevarious noise sensors are referred to herein as noise channels. It ispreferred that the noise channels contain none of the signal transmittedfrom the transmitter package 20.

A plurality of noise sensors 40 may be positioned at various distancesfrom the drill rig 10. Physical spacing tends to reduce the amount oftransmitted signal detected by sensors 40. The sensors 40 may bepositioned near sources of noise such as power lines, motors,generators, and pipelines to more effectively detect noise from suchsources. At least one sensor may be placed away from such manmade noisesources to detect magnetotelluric noise. In a preferred embodiment, thesensors 40 are magnetometers or include a magnetometer and an electricalfield or current detector. In a more preferred embodiment, the sensors40 include three-axis magnetometers and beam steering means, asdescribed in detail below. By proper selection of sensor type and byproper positioning, physically or by beam steering, the sensors canprovide a noise channel with minimum signal.

One or more sensors 42 may be mounted on the drill rig 10 to detectnoise. These sensors may include current detectors for detecting drivecurrents in motors such as motor 18 or output currents of electricalgenerators which provide current to the motors. The sensors 42 maypreferably include magnetometers as discussed above. In one embodiment,sensors 42 may include motion sensors, e.g. seismometers, which detectphysical motion, e.g. vibration, in various parts of the drill rig 10and equipment which drives the drill pipe 12. The sensors 42 may beattached to structural members of rig 10 or placed on floor members 11.In addition, sensors 42 may be coupled to the earth near support membersof rig 10 to detect earth motion induced by the rig 10.

As discussed above, the signal processor 28 receives a signal channelfrom amplifier 36 and also receives one or more noise channels fromvarious noise detectors 40 and 42. As discussed in the backgroundsection above, the processor 28 may include bandpass filters on allchannels which block all signals outside the frequency band in which theEM transmitter 20 operates. In addition, the processor 28 includes oneor more noise cancellers which, by reference to the noise channels,remove noise from the signal channel.

With reference to FIG. 2, a preferred noise canceller 44 will bedescribed. A preferred noise canceller 44 includes an adaptivetransversal filter 46. The canceller 44 has two inputs, a primary, i.e.signal channel, input 48 and a reference, i.e. noise channel, input 50.The adaptive filter 46 has a reference input forming the reference input50 of the noise canceller 44 and has an output 52 providing anapproximation of the noise contained in the signal on canceller 44 input48. The canceller 44 also includes an adder 54 which removes theestimated noise on filter 46 output 52 from the primary input on line 48to form an error signal e(t) on line 56. The error signal is fed back tofilter 46 and also forms the output of canceller 44 which is a noisefree signal, or at least an approximation thereof, having improvedsignal to noise ratio.

For best results, the noises in the primary input 48 and in thereference input 50 must be correlated and the reference input should befree of the signal. The object is to use the reference input to reducethe noise in the primary input. To the extent that the noise channelincludes desired signal, the canceller will cancel part of the signal.

For the purpose of illustration, assume that the primary input 48 is theup link electric field signal received by an EM telemetry system throughleads 30 (FIG. 1) and that this signal has been corrupted by noiseinduced by rotation of drill pipe 12. This signal therefore has thefollowing form:

f(t)=s(t)+n 1(t)

where the received signal, f(t), is the sum of the electric fieldcomponent of the telemetry signal, s(t), plus the electric field noisecomponent, n1(t), induced, for example, by drillstring 12 rotation. Theprimary signal may be sampled at regular intervals, T, and digitized toproduce the following discrete-time signal:

f _(i) =s _(i) +n 1 _(i),

where i refers to the sample number, from an arbitrary time origin,common to all measurements.

The reference input may be expressed in discrete time as:

y _(i) =n 2 _(i)

Where n2 _(i) is the reference noise signal which is assumed to becorrelated with the primary noise signal, n1(t), corrupting thetelemetry signal. This noise reference can be obtained using amagnetometer or an electric field sensor at a point sufficiently removedfrom the location where the primary signal is received so that there isno appreciable component of the telemetry signal in it. The correlationbetween n2 and n1 can be exploited to minimize the noise in the primaryinput. In general, the exact nature of this correlation need not beknown in advance for this noise cancellation method to work.

With this notation, noise cancellation is seen to be simply the jointprocess estimation problem whose structure is shown in FIG. 2. Theadaptive joint process estimation algorithm will be able to exploit thecorrelation between the two input signals to minimize the mean-squareerror, E[e(t)²], between f(t) and an estimator of the noise, n3(t),where:

 e(t)=f(t)−n 3(t)

or in discrete form,

e _(i) =f _(i) −n 3 _(i)

Taking into account the assumption that n2(t), and hence n3(t), areuncorrelated with s(t),

E[e(t)]=E[s(t)]+E[(n 1(t)−n 3(t))]

or

E[e _(i) ]=E[s _(i) ]+E[(n 1 _(i) −n 3 _(i))]

where E[ ] a denotes expected value of the quantity in brackets [ ].Adjusting the adaptive filter such that the mean squared value ofE[e_(i)] is minimum results in n3(t) being the best estimator of n1(t).

In its simplest embodiment, this invention uses an adaptive filter toapproximate the transfer function between a reference electromagneticnoise signal picked up by a magnetometer, e.g. sensor 40 of FIG. 1, andelectromagnetic noise contaminating the telemetry signal by minimizingthe mean-squared error between them. The telemetry noise approximationderived from an adaptive filter is subtracted from the noisy telemetrysignal to get a “noise-free” telemetry signal, or at least a signal withimproved signal to noise ratio.

FIG. 3 provides a more detailed block diagram of an EM telemetry systemand a model of signal and noise channels. Original data, d(t), isdigitized, encoded, modulated and then radiated as a telemetry signalinto the earth-pipe electromagnetic transmission channel by theelectromagnetic transmitter (E/M XMTR), 58, e.g. part of the electronicspackage 20 of FIG. 1. The electromagnetic telemetry signal istransmitted uphole via the earth-pipe transmission channel where it ispicked up as a difference signal between the borehole casing 32 at thesurface and earth electrode 34 (FIG. 1). The earth-pipe-electrodetransmission channel 60 is represented as a transfer function G2(s)which results in a signal s(t) being received at the surface location.The telemetry signal detected by the electrodes at the surface iscontaminated by electromagnetic noise sources near the surface such asmachinery (primarily on the drilling rig) and power lines. The transferpath 62 between the reference noise source, n2(t), and the telemetrynoise, n1(t), is denoted as transfer function G1(s) which results innoise n1(t) reaching the signal detector. In FIG. 3, an adder 64 is usedto model the combination of the transmitted data d(t) and the EM noisesource n2(t) to form the signal channel 66, s(t)+n1(t), which is theprimary input to the noise canceller. The combination actually occursbecause the signal sensor, e.g. the voltage detected between casing 32and electrode 34, detects both the signal, s(t), and noise, n1(t).

The output 68 of a magnetometer 70 forms the noise channel or referenceinput, n2(t), into a noise canceller 72 including an adaptive filter 74.Both the signal channel 66 and noise channel 68 may be converted todigital form by analog to digital converters 76. The adaptive filter 74transforms the reference noise signal, n2(t), into an approximationn3(t) of the telemetry noise n1(t) at its output 78. The differencebetween the filter's output 78 and the noisy telemetry signal 66 isproduced by subtractor 80 and is used as the error signal, e(t), intothe adaptive filter input 82, which also forms the output of thecanceller 72. The adaptive filter minimizes the error signal byadjusting its output to be as close an approximation (in the mean squaresense) to the noisy telemetry signal as possible. Since the referencenoise input, n2(t), is a function of the telemetry noise, n1(t), but nota function of the telemetry signal, s(t), and since the signal and noiseare not correlated, the filter can only force the reference toapproximate the telemetry noise, but not the telemetry signal. Theresult of the process is that the error signal, e(t), is anapproximation of a noise free signal s(t). This improved signal, i.e.the approximation of a noise free signal s(t), is coupled to a receivermodule 84 for further processing to reconstruct the original transmitteddata d(t). If the signals into noise canceller 72 were converted fromanalog to digital form by converters 76, a digital to analog converter86 may be used to convert the output 82 of canceller 72 back to analogform for receiver 84.

FIG. 4 provides a schematic of the adaptive filter 74 of FIG. 3. Thedigitized input signal 88 (e.g. a noise channel from a magnetometer) isrun through a series of unit time delays 90 of T seconds, eachdesignated as Z⁻¹. The signals are “tapped off” after unit time delayand each multiplied in multipliers 92 by unique filter tap coefficientsC₁, C₂, . . . , C_(n). The output of the filter is formed by summingtogether the gain-adjusted tap signals at the outputs of multipliers 92in summer 94. The filter's transfer function is determined by the valueof the filter's tap coefficients. The filter's transfer function isadapted by changing the values of the filter tap coefficients C₁, C₂, .. . , C_(n).

FIG. 5 shows the filter's adaptation algorithm for one of the filter'scoefficients. The tap coefficients are updated after every “shift”(every T seconds) of the digitized reference signal through the filter'stapped delay line. The coefficient at the j^(th) tap is updated by avalue equal to the respective tap signal, y(T−j ) times the canceller'sdigitized error signal, e(T), times a small adaptation coefficients, β.The adaptation algorithm may be represented by the following equation:

cj _(i+l) =cj _(i) +β·e _(i) y _(i−j)

For an adaptive filter to work best, the noise reference, or noisechannel, would contain only noise and not contain any of the desiredsignal. In real systems, some of the desired signal will be detected byany EM detector used for detecting a noise source. Prior systems placenoise detectors near noise sources to improve the noise channel, i.e.increase the noise to signal ratio in the noise channel. In similarfashion, the noise canceller will work better if the signal channel hasas little noise in it as possible, i.e. there will be less noise toremove. As with noise detectors, it is known to select positions forsignal detectors where the maximum signal will be detected and theminimum noise will be detected. In certain embodiments of the presentinvention, one or more magnetometers are preferred for detecting EMsignal and/or noise. A three-axis magnetometer and beam steeringtechniques may be used to provide a noise channel with minimum signalcontent and/or a signal channel with minimum noise content. A three-axismagnetometer is essentially a set of three magnetometers positionedorthogonally to each other with each magnetometer having a separateelectrical output representing the magnetic field in its respectivedirection.

Both the transmitted EM signal and noise originate as vector fields. Itis possible to receive three different components of each field,electric and magnetic, and use these components to fully identify thevector. In a preferred embodiment, the electric field would be measuredas shown in FIG. 1. Each of the detectors 40 and 42 would include athree-axis magnetometer which measures three components of the magneticfield, two components being parallel to the surface of the earth, andthe third component being orthogonal to the surface of the earth. Callthese three components H_(x), H_(y), and H_(z) respectively.

As an example of the use of these components, suppose the downhole EMtelemetry transmitter, e.g. package 20 of FIG. 1, is an electric fieldtype of transmitter and suppose the wellbore is nearly vertical. Twotechniques are commonly employed in the operation of E-fieldtransmitters. In one of the techniques, an electric current is launchedinto the formation and into the drill pipe using a toroidal coil woundaround a section of the drill collar. The other technique is to apply avoltage across an insulating gap. In either case, a current is launchedalong the drillstring and into the formation. The component of themagnetic field of the received signal at the earth's surface arisingfrom the current launched into the drillstring is parallel to theearth's surface. This is because the top of a borehole is alwaysorthogonal to the earth's surface, the current flows in the direction ofthe borehole, and the magnetic field due to a current is orthogonal toits direction of flow. When the portion of the well in which thetransmitter is situated is vertical, the magnetic field received at theearth's surface which has propagated through the earth will also tend tobe parallel to the earth's surface. This is because the fieldpropagating through the earth will resemble that due to an electricdipole transmitter with the dipole axis oriented along the boreholeaxis. In this case, the magnetic field is always orthogonal to thedipole axis, and thus parallel to the earth's surface. It is clear inthis case that the in-band, horizontal plane magnetic signals can beused to enhance the telemetry signal picked up using an electric fieldsensor, while the vertical component of the magnetic field, H_(z), canserve as a noise reference, assuming a source of electric field noise iscorrelated with the vertical component of the magnetic field,independent of the signal. A single three-axis magnetometer can be usedin this case. The vertical component of the magnetic field serves as thenoise channel while some linear combination of the electric field andthe horizontal components of the magnetic field serves as the signalchannel.

If the electric field detector is sufficient for the signal channel, asingle, vertically oriented magnetic receiver can be used for the noisechannel in such an application. A single vertical magnetometer tends tonot detect the signal because its magnetic field is horizontal, so thatits output would be primarily due to noise sources. Thus, when thedirection of the magnetic field component of an EM signal or noise isknown, or predictable, a single magnetometer physically positioned toprovide a signal channel or a noise channel is a preferred detector. Inonshore, i.e. on land, drilling operations, the electrical component ofthe transmitted EM signal is usually stronger, whereas in offshoredrilling the magnetic component of the transmitted signal is usuallystronger. Therefore, it is preferred to use a magnetometer as a noisedetector onshore and as a signal detector offshore.

In many cases, the magnetic fields produced by the EM transmitter andvarious noise sources will not be aligned as discussed above. That is,the transmitted signal may produce a magnetic field which is not exactlyhorizontal, e.g. when drilling deviated holes. Likewise, some sources ofelectric field noise will produce magnetic field noise having apredominately horizontal component. This leads to two sensorarrangements, either or both of which may be used in embodiments of thepresent invention. Generally, these two arrangements both use athree-axis magnetometer and beam steering of the three outputs of themagnetometer. In a first case, the beam steering is used to align thedetector with a noise source and, in a second case, it is used to alignwith the signal source. A single three-axis magnetometer may be used forboth purposes simultaneously.

In the first case, all three outputs of a three-axis magnetometer mayrespond only to the noise, or at least much more noise than signal, ifthe magnetometer is sufficiently remote from the telemetry system andthe origin of the noise is not local to the E-field receiver (if it is,the E-field receiver should be moved). The magnetometer outputs can becombined into a single signal which effectively simulates a single-axismagnetometer oriented in the direction of the noise. This output can beused as a noise reference or noise channel, as described earlier.

In the second case, all three outputs of a three-axis magnetometer mayrespond primarily to the signal. The magnetometer outputs can becombined into a single signal which effectively simulates a single-axismagnetometer oriented in the best direction for reception of the signal.The electric field signal can be used as a reference for steering athree-axis magnetometer, and can be further combined with themagnetometer output as an additional signal processing step.

The procedure of combining the magnetometer outputs to simulate a singleaxis magnetometer for these two cases is referred to as “beam steering.”Specific examples of beam steering a three axis detector to provideimproved noise and signal channels are provided below.

Beam Steering Magnetometer in Direction of Noise

FIG. 6 illustrates the apparatus and method used for beam steering adetector in the direction of EM radiation from a noise source for thefirst case. In this figure the EM telemetry system uses an E fielddetector 96, e.g. casing 32 and electrode 34 of FIG. 1, to detect thesignal transmitted by an EM transmitter, e.g. electronics package 20 ofFIG. 1. A noise channel is provided by a three-axis magnetometer 98comprising magnetometers 100, 102 and 104, positioned orthogonally toeach other. The outputs 106, 108 and 110 of the magnetometers 100, 102and 104 are coupled through filters 112 to multipliers 114 where theyare multiplied by coefficients α, β, and γ. The outputs of themultipliers 114 are combined in adder 116, which provides a noisechannel at its output 118. The output 120 of signal detector 96 is alsocoupled through a filter 112 to the positive input of an adder 122. Thenoise channel 118 is coupled to the negative input of adder 122. Theoutput of adder 122 is the input to an algorithm represented by box 124,which produces the coefficients α, β, and γ which are coupled to themultipliers 114.

A least squares technique is used to determine three coefficients, α, β,and γ, such that:

α·H(band_pass_filtered)_(x) +β·H(band_pass_filtered)_(y)+γ·H(band_pass_filtered)_(z) ≈E(band_pass_filtered)

Given that the magnetic field measurements do not contain the signal (orcontain much more noise than signal), this effectively points themagnetometer toward the noise. To see why this is so, consider a noisesource {right arrow over (N)} coming from a specific direction{circumflex over (n)}, where |{circumflex over (n)}|=1, and consider asignal derived from the three magnetometer outputs given by

z=a·H _(x) +b·H _(y) +c·H _(z)

At what values of a, b, and c will the magnitude of z be maximized? Notethat If {circumflex over (n)} is of the form

{circumflex over (n)}=α·î+β·ĵ+γ·{circumflex over (k)}

then

z=(a·α+b·β+c·γ)·|{right arrow over (R)}|

or

z={right arrow over (A)}·{circumflex over (n)}·|{right arrow over (N)}|

where {right arrow over (A)}=a·î+b·ĵ+c·{circumflex over (k)}

By one of the basic properties of the inner product, this is maximizedwhen {right arrow over (A)} is aligned with {circumflex over (n)}.

The fitting of this combined output to the electric field signalguarantees that the magnetometer is steered toward the common noisesource to which both instruments are responding. That is, at theappropriate values of α, β, and γ, the three-axis magnetometer issynthetically shifted to the direction of the common noise source. Inpreliminary testing of this concept, a simple linear least squares fitof the three magnetometer outputs over about 10 seconds of data wassufficient to determine the coefficients α, β, and γ. This is preferablydone at a time when the downhole transmitter is not operating.

The bandpass filters 112 of FIG. 6 are optional, but desirable. If theyare used, they should be identical. In addition, any signal samplingshould be synchronized for all four signals. The output 118 of thissystem can be treated as a noise reference and used with an adaptivenoise canceller as discussed above. Alternatively, the three outputsignals from the magnetometer can first be processed using an adaptivenoise canceller, and the resulting three noise estimators can then besynthetically steered to optimize reception of the noise.

Beam Steering Magnetometer in Direction of Signal

FIG. 7 illustrates apparatus which may be used for steering the outputsof a three axis detector in the direction of signal. The apparatus maybe identical to the apparatus of FIG. 6 and the same reference numbersare therefore used to identify the various parts. The main differencebetween FIG. 6 and FIG. 7 is in the positioning of the three-axismagnetometer 98. In FIG. 6 the magnetometer 98 is positioned to detectprimarily noise, but in FIG. 7, it is positioned to detect primarilysignal in at least two of the magnetometers 100, 102 and 104. As in FIG.6, the E field detector 96 of FIG. 7 detects the transmitted telemetrysignal.

Two methods can be employed to effectively steer an output derived froma three axis magnetometer in the direction of the signal. In the firstmethod, the adder 122 and the algorithm 124 are not needed. Instead, thecoefficients α, β, and γ are treated as direction cosines and calculatedbased on the anticipated arrival direction of the transmitted telemetrysignal. The arrival direction of the signal does not necessarilycorrespond with the direction from the magnetometer package to thesignal source, i.e. more than simple geometric calculations arerequired. The arrival direction is the direction of the magnetic fieldlines at the earth's surface arising from the EM telemetry transmitter.This direction can be estimated analytically using Maxwell's equationsgiven the location of the source, its orientation and the location ofthe magnetometer package. In the above discussion of use of amagnetometer (without beam steering) to detect noise, it was assumedthat for an Electric Field transmitter oriented vertically, the magneticfield will be in the horizontal plane. A more detailed analysis revealsthat the magnetic field lines arising from the transmitter will, at theearth's surface, point along the tangent to a circle, the center ofwhich is at the vertical projection of the transmitter to the surface,the circumference of which passes through the magnetometer package, andthe tangent of which is projected from the magnetometer package.

A second method of steering the three-axis magnetometer may be used whenthe magnetometer signals are not significantly affected by noisecorrelated with the noise detected by the electric field sensor, i.e.when they detect primarily signal. In this case, the magnetometer can besteered in the direction of greatest correlation with the electric fieldsensor, which will be the direction of best signal detection. Thetechnique for doing this is the same as the algorithm described abovewith respect to FIG. 6. The algorithm causes the alignment of themagnetometer 98 with signal in this case because the magnetometer isdetecting primarily signal. Some further noise improvement can beachieved by adding the signal output of FIG. 7 with the E-field signalsince random components will tend to cancel each other.

In some cases it is possible to use one three-axis magnetometer 98 andtwo sets of multipliers 114, each having a different set of coefficientsα, β, and γ to provide both a signal channel and a noise channel. Thiscan occur when the detector is positioned so that the detector respondsprimarily to signal in one direction and primarily to noise in another.Normally this will require prior knowledge of relative locations of thesignal transmitter, the noise source and the detector. Then basicgeometric calculations can be made to obtain the appropriatecoefficients α, β, and γ for signal channel and for the noise channel.

The location of the transmitter is normally known, so that it isrelatively simple to estimate the direction of signal fields. When it isknown, but the location of the noise source is not known, the noisesource direction can be measured using the method of FIG. 6 when thetransmitter is not operating.

A very simple case of selecting beam steering coefficients occurs whenthe signal magnetic fields are horizontal and the noise fields arevertical. In that case, which was discussed above, the verticalmagnetometer, H_(z), would be used only for the noise channel. This isequivalent to setting the coefficients α, and β to zero for the noisechannel. Some combination of the two horizontal magnetometers wouldprovide the signal channel. This is equivalent to setting thecoefficient γ to zero and selecting appropriate values for α, and β toprovide a signal channel.

While the embodiments shown in FIGS. 6 and 7 include a magnetometer as athree-axis detector, directional E field detectors, i.e. antennas, couldalso be used. The antenna could be a single direction antenna alignedwith the signal or noise E field or could be a three-axis antenna. Aswith the three-axis magnetometer, a three-axis antenna would comprisethree directional antennas positioned orthogonally to each other andwould provide three outputs. The above described beam steeringtechniques apply to such E field detectors. Such antennas may beparticularly useful as a noise detector in offshore applications whereit is preferred to use a magnetometer, single or three-axis, as thesignal detector. They may be useful for signal detection in onshoreapplications where the signal us usually more easily detected as an Efield.

The various magnetometer detectors discussed above provide the advantageof a signal channel with minimum noise and/or a noise channel withminimum signal. Such improved signal and noise channels provide improvedinputs to a noise canceller, e.g. canceller 72 of FIG. 3, and allow itto work more effectively. As discussed with reference to FIG. 1,detectors 42 may preferably include motion sensors or otherelectromechanical transducers such as seismometers. Since such detectorscan be shielded so that they do not detect any EM signal, they canprovide a noise channel free of transmitted EM signal. This use ofelectromechanical transducers as EM noise channel detectors resultedfrom our discovery that hitting the side of a land drilling rig producesa response, i.e. noise, in an electrical field sensor. We believe thatthere are several mechanisms which explain why physical motioncorrelates to EM noise.

As any part of the drill rig 10 vibrates, it cuts the earth's magneticfield lines and thus by Faraday's law (induced EMF is proportional torate of change of magnetic flux), creates an electric field. Where it ispossible to complete an electric circuit, the electric field creates acurrent, and hence another magnetic field. Any time varying electricfield creates a magnetic field and vice-versa. The fact that a currentcreates a magnetic field is simply a manifestation of this samephenomenon, but is distinguished in this case because the magnetic fieldarising directly from the current will generally be stronger than themagnetic field arising simply from a time varying electric field. Hence,any vibration can be expected to correlated with electric and magneticnoise.

Any joint between dissimilar metals will produce an electromotive force.As the rig 10 is stressed, the effects from joints of dissimilar metalson the rig will vary as the contact resistance changes. In addition, therig itself can act as an antenna in picking up electromagnetic energy.Rectifying joints can demodulate high frequency radiation, resulting inlower frequency currents having a DC component being induced on the rigand acting as a noise source due to variations in the joint as the rigis stressed by vibration.

As a drillstring is rotated in the earth's magnetic field, currents areinduced in the drillstring as a consequence of Faraday's law (inducedEMF is proportional to rate of change of magnetic flux). The amount ofcurrent will vary as the contact of the drillstring and bit with theformation varies. This serves as both a source of electrical andmagnetic interference and can be correlated with drillstring rotation,i.e. physical motion.

There is also some reason to expect that some of the noise will becorrelated with flowing fluids. It is well known that a streaming fluidcontaining clay particles creates an electromotive force. Variations inflow will thus manifest themselves as variations in the electric field(and where it is possible for currents to flow, as variations in themagnetic field). See e.g. P. 525 of Physical Chemistry, Second Edition,William F. Sheehan,1970, Allyn and Bacon, Inc., Boston. This referencealso mentions another effect known as the Dorn effect, which can producea potential difference with a flowing fluid containing clay particles(e.g. drilling mud).

In addition, shale and most minerals conduct electricity. Thus, as thebit contacts the formation, an EMF is developed due to the dissimilarmaterials. The chemical action between the drilling mud, formationfluids and the drillstring is capable of creating an electromotive forcewhich can be modulated by vibration. Thus, EM noise created by theseelectrical phenomena may correlate with vibration in the drill pipe 12.

Other types of electromechanical transducers can also provide anelectrical signal representing mechanical forces correlated with theseeffects. While a vibration detector can detect motion in the drill rig10, the motions will also cause variations in stress of the rig memberswhich can be detected by a strain gauge connected to the rig. While flowlines may produce detectable vibrations, the flow and variations in theflow can also be detected by flow rate meters and pressure detectorscoupled to the flow lines.

The noise canceling systems described above with respect to FIGS. 1through 5 each have a single reference or noise channel input. Theimproved noise detectors described herein can provide a number of noisechannels, each of which may desirably be removed from the signalchannel. FIGS. 8 and 9 illustrate systems for removing multiple noisesources.

In FIG. 8, there is shown three noise channels, labeled A, B and C.There may, of course, be more than three noise channels. Each channel iscoupled through a filter 126 to an adder 128 which provides a combinednoise channel to the noise channel input 130 of an adaptive filter 132.Adaptive filter 132 may comprise the noise canceller circuitry 72 ofFIG. 3. The signal channel is coupled to primary input 134 of adaptivefilter 132. The signal with improved signal to noise ratio is providedon output 136.

Filters 126 preferably each include a bandpass filter to block anyfrequencies outside the operating range of the EM transmitter whichgenerates the desired signal. They also preferably have transferfunctions which adjust amplitude, and possibly phase, in accordance withthe transfer function by which the various noise sources are coupled tothe signal channel detector. These adjustments to the noise channelswill help the adaptive filter 132 properly remove the noise from thesignal channel.

FIG. 9 illustrates a system in which a separate adaptive filter is usedto remove each noise source from the signal channel. In FIG. 9 threenoise channels, A, B, and C are each coupled through filters 138 toseparate adaptive filters 140, 142 and 144, each of which may comprisethe noise canceller circuitry 72 of FIG. 3. In this case, the filters138 would provide only band pass filtering to remove frequencies outsidethe operating range of the EM transmitter which generates the desiredsignal. It is not necessary to adjust amplitude and phase of the noisechannels since the adaptive filters will operate on each one separately.

Noise channel A is coupled to the noise channel, or reference, input 146of adaptive filter 140. The signal channel is coupled to the primaryinput 148. An improved signal from which the noise reference on noisechannel A has been removed is provided on the output 150 of adaptivefilter 140.

Noise channel B is coupled to the noise channel, or reference, input 152of adaptive filter 142. The output 150 of adaptive filter 140 is coupledto the primary input 154 of adaptive filter 142. An improved signal fromwhich the noise reference on noise channel B has been removed isprovided on the output 156 of adaptive filter 142. Since filter 140 hasalready removed noise channel A from the signal, the improved signal onoutput 156 has both noise channels A and B removed.

Noise channel C is coupled to the noise channel, or reference, input 158of adaptive filter 144. The output 156 of adaptive filter 142 is coupledto the primary input 160 of adaptive filter 144. An improved signal fromwhich the noise reference on noise channel C has been removed isprovided on the output 160 of adaptive filter 144. Since adaptivefilters 140 and 142 have already removed noise channels A and B from thesignal, the improved signal on output 160 has all three noise channelsA, B and C removed.

While phase shifting of the noise channels is not needed in the normalsense, certain time delays are needed. In FIG. 9, noise channel B iscoupled through a time delay 162. This time delay is set to compensatefor the delay in the signal channel as it passes through the adaptivefilter 140. This delay 162 keeps the noise channel B synchronized withthe signal channel at the inputs 152 and 154 to adaptive filter 142. Fordigitized signals, this means that the delay 162 may be simply a oneclock cycle delay.

In similar fashion, a delay 164 is provided for noise channel C. Delay164 is set to compensate for the time delays through both adaptivefilters 140 and 142. This delay 164 keeps the noise channel Csynchronized with the signal channel at the inputs 158 and 160 toadaptive filter 144. For digitized signals, this means that the delay164 may be simply a two clock cycle delay.

As noted above, there may be more than three noise sources havingsufficient effect on the signal channel to warrant noise cancellerapparatus. The FIG. 9 apparatus may be expanded to include a separateadaptive filter for each noise source.

In the FIG. 9 embodiment, it is preferred that the noise sources beranked in order of significance, with the most significant usually beingthe noise having the greatest magnitude. The most significant should becoupled to the first adaptive filter. Thus, in FIG. 9 noise channel Awould be the most significant and noise channel C would be the leastsignificant. This arrangement removes the biggest noise source first andshould improve the efficiency of the later adaptive filters which willremove smaller noises.

The significance of various noise sources will not be the same at allwell sites. It may also change during the drilling of a well. It istherefore preferred to use an algorithm which actively selects the bestorder in which the noise channels should be removed from the signal. Ifsignificance is based only on magnitude, the algorithm can simplymeasure amplitude of each noise channel over a period of time and rankthe noise channels by amplitude. The ranking can be done during aninitial setup of the system and, if desired, repeated on a regular basisduring drilling operations.

Not all noise channels will be of the same quality in terms of noise tosignal ratio. The motion sensors discussed above may provide a noisechannel containing essentially none of the transmitted signal. Thiswould be a high quality noise channel because it allows a noisecanceller to remove a noise without also reducing the signal level. Evenif the magnitude of such a noise channel is less than other noisechannels, it may be considered the most significant and coupled to thefirst adaptive filter since it will have no negative effect on thedesired signal.

In similar fashion, some high amplitude noise channels may be ratedlower in significance for other reasons. For example, the directionalsensors disclosed herein may provide a signal channel which effectivelyexcludes some noise sources. While a noise sensor may provide a strongnoise channel for such a noise source, there is no need to provide thechannel to a noise canceller since the signal channel does not containthat noise.

In most cases, the noise and signal channels will be digitized as shownin FIG. 3. All processing after the digitization is normally done by acomputer programmed to perform the filtering, summing, subtracting, etc.functions. The algorithm for ranking noise channels will also beperformed by software. This allows the ranking algorithm to be performedon a real time basis and allows reordering of the noise channels on areal time basis.

It is apparent that various changes can be made in the apparatus andmethods disclosed herein, without departing from the scope of theinvention as defined by the appended claims.

What we claim as our invention is:
 1. Apparatus for removing noisegenerated by multiple sources from a received signal in a boreholeelectromagnetic telemetry system comprising; first and second noisesensors detecting first and second noise sources and providing first andsecond noise channels representative of said noise sources, first andsecond adaptive filters, having reference inputs coupled to said firstand second noise channels respectively, and a first time delay unitcoupling said second noise channel to said second adaptive filterreference input, said first adaptive filter having a primary inputcoupled to said received signal, and an output coupled to the primaryinput of said second adaptive filter.
 2. Apparatus according to claim 1wherein: said first time delay unit delays said second noise channel byabout the time required for the received signal to propagate throughsaid first adaptive filter.
 3. Apparatus according to claim 2 furthercomprising: a third noise sensor detecting a third noise source andproviding a third noise channel representative of said third noisesource, a third adaptive filter, having a reference input coupled tosaid third noise channel, said third adaptive filter having a primaryinput coupled to the output of said second adaptive filter.
 4. Apparatusaccording to claim 3 further comprising: a second time delay unitcoupling said third noise channel to said third adaptive filterreference input.
 5. Apparatus according to claim 4 wherein: said secondtime delay unit delays said third noise channel by about the timerequired for the received signal to propagate through said first andsecond adaptive filters.
 6. Apparatus for removing noise generated by aplurality of noise sources from a received signal in a boreholeelectromagnetic telemetry system comprising; a plurality of noisesensors detecting a plurality of noise sources, a plurality of adaptivefilters, each having a reference input coupled to one of said noisesensors, and time delay units coupled to the reference inputs of thesecond and subsequent adaptive filters delaying said signals from thenoise sources to maintain synchronization between said noise sources andsaid received signal, said adaptive filters coupled in series, with thefirst filter having a primary input coupled to said received signal andthe last filter providing an improved signal from which noise from saidnoise sources has been substantially cancelled.
 7. Apparatus forremoving noise generated by a plurality of noise sources from a receivedsignal in a borehole electromagnetic telemetry system comprising; aplurality of noise sensors detecting a plurality of noise sources, aplurality of adaptive filters, each having a reference input coupled toone of said noise sensors, said adaptive filters coupled in series, withthe first filler having a primary input coupled to said received signaland the last filter providing an improved signal from which noise fromsaid noise sources has been substantially cancelled, said noise sourcesare ranked in order of significance, with the most significant coupledto the first adaptive filter and the least significant coupled to thelast adaptive filter.
 8. The apparatus of claim 7 wherein the order ofsignificance is based on the amplitude of noise channels produced bysaid noise sensors.
 9. The apparatus of claim 8 wherein the noisechannel having the greatest amplitude is considered the mostsignificant.
 10. The apparatus of claim 7 wherein the order ofsignificance is based on the magnitude of each noise source which iscontained in the received signal.
 11. The apparatus of claim 10 whereinthe noise channel generating the greatest noise amplitude in the signalis considered the most significant.
 12. The apparatus of claim 7 whereinthe order of significance is based on the magnitude of signal which iscontained in each noise channel.
 13. The apparatus of claim 12 wherein:the noise channel containing the least signal is considered the mostsignificant.